The subject invention is directed to fiber optic links for radar systems, and is directed more particularly to light controlled impedance matching techniques for microwave systems using optoelectronic feedback and control techniques.
In order to fully utilize the potential of advanced microwave systems with their vast data collection and processing capabilities, fast and very efficient data communication transfer systems must be utilized. RF fiber optic links utilizing optoelectronic components have the capability of meeting such communications requirements for several reasons. Fiber optic links are not susceptible to radio frequency noise and are capable of operating at very high data rates and at high radio frequencies without creating opportunities for interception and detection. The light weight and small size of fiber optic cables enables the use of highly redundant paths between units and provides for easier mechanical routing in corporate feeds of electronically scanned arrays, thus improving reliability and damage tolerance. Finally, the EMI immunity of fiber optics reduces equipment failures caused by electrical power transients and could also improve the capabilities and reduce the size and weight of radar and other avionic systems.
Typically, a fiber optic link includes a photonic transmitter responsive to an input RF signal for producing an RF amplitude modulated light signal. The modulated light signal is communicated via a fiber optic cable to a photonic photodiode detector receiver that produces an electrical RF output in response to the received modulated light.
A consideration with present fiber optic links is the typically high level of insertion loss introduced by a fiber optic link. A significant factor in the high level of insertion loss is the typical technique of providing broadband 50 ohm impedance matching at the RF input and RF output of the fiber optic link for compatibility with microwave circuits, usually by the addition of resistance. While such impedance matching provides for reduced reflections over a relatively broad bandwidth, it results in substantial power loss.
A further consideration with fiber optic links in general is the lack of consistency in the impedance characteristics of laser diodes utilized in photonic transmitters. There is also a lack of consistency in the impedance characteristics of the photodiodes utilized in photonic receivers, although to a lesser degree.